Conventionally, research has been conducted to enhance heat resistance of printed wiring boards used in electronic devices, by improving, for example, glass transition temperature (Tg). Research also has been conducted to enhance flame retardancy of these boards. As a recent trend, compact electronic devices such as mobile devices have been made smaller and thinner with more functions. Along with this trend, there is an increasing demand in the market for printed wiring boards that have a lower dielectric constant and a lower CTE (coefficient of thermal expansion). As an insulating material for printed wiring boards, an epoxy resin composition is commonly used. The epoxy resin composition contains a phenol-based curing agent, a diamine-based curing agent, a cyanate-based curing agent, or an acid-anhydride-based curing agent, for example, as a curing agent for epoxy resin. Among these various curing agents, the acid-anhydride-based curing agent is known to be particularly effective in lowering dielectric constant. Examples of conventional acid-anhydride-based curing agents include polyfunctional acid anhydride compounds that contain a plurality of cyclic acid anhydrides per molecule and styrene-(maleic acid) (SMA) copolymers. In Unexamined Japanese Patent Publication No. 9-194610, for example, a copolymer essentially composed of styrene and maleic anhydride (SMA) is used as the acid-anhydride-based curing agent.